1) Quit Steam
2) Find the "package" directory inside of the main Steam install directory (typically C:\Program Files (x86)\Steam on Windows, ~/.steam/steam on Linux)
3) Inside there create a file called "beta" (no extension), with the following content: steampal_stable_9a24a2bf68596b860cb6710d9ea307a76c29a04d
4) Reopen Steam with the -gamepadui argument
Fair warning: it's a little bit janky when it's not running on Steam Deck hardware. At least it was when I tried it out, but it'll give you a pretty decent idea what it's like. Also to undo it just delete that file and relaunch Steam without the extra argument.